The use of gas fed electrodes in various electrolytic processes is well-known in the art. The structure and use of such electrodes are demonstrated in the following U.S. patents among others: U.S. Pat. Nos. 4,229,490; 4,278,525; 4,213,833; 4,301,218.
Generally, such porous electrodes are used in systems capable of generating electricity, such as in fuel cells, and in electrochemical processes in which the electrode functions as a depolarized cathode, such as in chloralkali cells. Also of growing interest is the use of fuel-fed porous catalytic electrode structures in electrowinning metals such as copper from aqueous acidic solutions. In this regard see U.S. Pat. Nos. 3,103,473; 3,103,474; and 3,793,165 as examples of such processes.
In processes employing gas fed electrodes, the electrode typically is mounted in a vertical position in a cell and gas is fed to one side of the electrode while the other side is in contact with an aqueous electrolyte. The electrochemical reaction generally is believed to take place at the interface of the liquid electrolyte, the gas phase and the solid catalyst of the electrode. Since the three phase interface area is believed to be important in processes employing gas fed electrodes, means have been developed for maintaining the three phase interface within the passageways of the porous electrode bodies. One technique that has been employed is to utilize a hydrophobic material on the interior pore sufaces, particularly on the gas side of the electrode structure. The hydrophobic nature of this surface tends to prevent the electrolyte from wetting the structures and penetrating entirely through the electrode.
Another technique that is employed to maintain the three phase interface is to very carefully balance the gas pressure applied and the capillary pressure generated by the electrolyte solution. Generally, this is achieved by means of a porous electrode body which has a very narrow distribution of pore sizes.
In yet another technique, it is known to use a double porosity structure for the electrode body, such that the layer designed to be in contact with the electrolyte has significantly smaller pores than those pores which are in the layer adapted to be on the gas side of the structure. In this way it is possible to apply a gas pressure through the larger pore area that is greater than the median electolyte capillary pressure in the large pores, but smaller than that in the small pore layer so as to maintain the three phase contact sector within the passageways.
Unfortunately the foregoing techniques are not particularly satisfactory when the overall size of the gas fed electrode is designed to function vertically in large scale cells having relatively deep supply of electrolytes such as cells employed in electrowinning of metals from aqueous acidic solutions since problems arise in maintaining the necessary three phase interface inside the electrode over its entire depth. Basically, the hydraulic pressure of the electrolyte against the electrode increases with depth while the gas pressure normally remains constant. Consequently, there is a tendency for gas to percolate through the top of the electrode into the electrolyte and for liquid to penetrate into the gas compartment at the bottom. This tendency becomes more and more pronounced as the electrode size increases. As a result there is a net loss of gaseous reactant and the electrochemical functioning of a portion of the electrode is impaired by the penetration of the liquid into the electrode and gas compartment. Accordingly, there remains a need for an improved porous gas fed electrode structure, particularly for large scale applications, which will avoid the considerable disadvantage and difficulty of preventing gas percolation through such porous electrode structures.